1. Technical Field
This invention relates to the field of imaging displays, and more particularly to imaging display calibration.
2. Description of the Related Art
Imaging displays have become commonplace in the medical industry and are used in medical imaging systems such as magnetic resonance imagers, computer tomography devices, nuclear imaging equipment, positron emission tomography and ultrasound. With the adoption of imaging displays in such critical medical applications, the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) recognized an emerging need for a standard method addressing the transfer and presentation of images. Accordingly, the ACR and NEMA formed a joint committee to develop the Digital Imaging and Communications in Medicine (DICOM) standard.
DICOM Part 14 was developed to provide an objective, quantitative mechanism for mapping digital image values into a given range of luminance. Specifically, DICOM Part 14 specifies a standardized display function for display of grayscale images. More particularly, DICOM Part 14 defines a relationship between digital image values and displayed luminance values based upon measurements and models of human perception over a wide range of luminance. DICOM Part 14 further specifies calibration parameters that are used to calibrate emissive display systems.
When calibrating a display, a characteristic curve of the display's characteristic luminance response is measured using a test pattern. The test pattern typically consists of a square measurement field comprising 10% of the total number of pixels displayed by the system. The measurement field is placed in the center of the display. A full screen uniform background surrounds the square measurement field. The background should have a luminance that is 20% of the display's maximum luminance.
Presently, display calibration is a time-consuming and inefficient process. As such, display calibration is error prone. Further, because of the time involved, display calibration is performed on a periodic basis, for example every six months, so as not to be too inefficient. A photometer can be manually held to the face of the display in the center of the measurement field. The display driving level (DDL) of the measurement field then is stepped through a sequence of different values, starting with zero and increasing at each step until the maximum DLL is reached. The luminance of the measurement field is measured by the photometer at each DDL and the luminance values recorded. The DDL is a digital value given as an input to a display system to produce a luminance. A plot of the luminance vs. DDL then generated to model the characteristic curve of the display system over the luminance range. The plot of the measured luminance characteristic curve is then compared to a grayscale standard display function.
To calibrate a display system, the luminance characteristics of the display system are adjusted to compensate for differences between the measured luminance characteristic curve and the grayscale standard display function. For example, the minimum and maximum luminance intensity can be adjusted using a display system's black and white adjustments. Further, some imaging systems are provided with display controllers which can provide an input-to-output correction through the use of a lookup table (LUT) to optimize the grayscale presentation. Such systems are typically provided with software that receives measured luminance values and compares the measured luminance values to the LUT to determine correction factors.
As noted, typical display system calibration cycles are six months. If a medical imaging system is not found compliant, an imaging center can undergo heavy fines. Further, repeat offenders can lose their operating license. In the case that a misdiagnosis is induced by a display which is out of calibration, a medical imaging center operating the display can be held legally responsible. Moreover, the medical imaging center would likely become entangled in costly litigation.